Big brother Instincts
by wolfypuppypiles
Summary: Prompt fill from tumblr Izzy is replaced by a demon and no one but Alec can tell the difference


When Izzy went missing on a mission, it wasn't immediately noticed to be a problem. Because another Izzy came back.

Alec had been killing the last shax demon in the small hoard they took down, squishing it under his boot, when he noticed his sister's absence.

"Izzy?" He usually kept a close eye on both his siblings but he'd been momentarily distracted. He cursed under his breath and spun around, searching for her as his heart thumped fast in panic. She had only been missing for a few seconds and already, his mind was racing with possibilities, all of them horrible.

He was about to call out again when she stepped out of the shadows at the other end of the alley.

"You miss me?"

Alec raced over and grabbed her by the shoulders, searching her for injuries. She looked fine, hunting gear perfectly clean.

"Where did you go? Don't go off like that, I almost had a heart attack."

Izzy only laughed and brushed his hands off but there was something stiff about her smile that put Alec off.

"Just taking care of one of the demons trying to get away. You got them all on this end?"

Jace came jogging over, wiping blood off his seraph blade.

"Yeah, we got them all. You guys wanna go out for lunch? Alec, you can call Magnus to come too if you want."

Izzy smiled in that same stiff way again and Alec squinted at her. He didn't trust the way she was looking at Jace as she answered his question. There wasn't any of that usual fondness, it was more like hunger, her eyes seeming to drink him in, as her smile became predatory. It was so unlike his sister that Alec took a step back and shivered.

He shook his head, pulling his seraph blade out.

"You aren't Izzy."

The fake Izzy gasped as Jace put an arm in front of her, pushing her slightly back.

"Alec. What are you doing?"

Alec didn't want to hurt to hurt something that looked and sounded like his sister, but if this Izzy was an imposter it meant his real sister was in trouble.

"That's not her. It's a demon."

Jace turned and looked at Izzy, seeing only the demon's glamour. He turned back to Alec and placed a hand on his chest, pushing him away from their fake sister.

"What are you talking about man. How hard did you hit your head when that demon pushed you?"

Alec groaned and shoved Jace away, pointing his blade at Izzy's throat as she stared at him.

"I'm fine, it's her that's the problem."

Alec didn't understand why Jace couldn't see it. The real Izzy would have disarmed him by now for one thing, and every emotion crossing her face had a slight amusement to it, as if it were a game she very much enjoyed playing.

Alec was so focussed on the demon in front of him he didn't see Jace coming until his blade was knocked from his hand and his arms were being pulled behind his back. He tried to fight him off but after only a few seconds, was pinned under his brother, being turned over and his hands being cuffed behind his back.

"I'm sorry, Alec. But I can't let you hurt our sister."

Alec pulled at his cuffs, grunting in annoyance as Jace pulled him up from the ground. He looked genuinely concerned as he reached a hand behind Alec's head, pressing into his hair to find where he'd hit his head. Alec had been shoved by a few of the shax demons, as small as they were, and hit his head on the alley wall, but he was fine. Apparently Jace didn't seem to think so.

Jace's hand came away wet with blood and he frowned.

"I think you have a concussion, we better get you back to the institute and checked out."

Alec pulled on his cuffs again and tried to shake off Jace's grip.

"I'm fine! Izzy's missing and this one's a demon, how can you not see that? It's nothing like her!"

Jace looked at Alec with the most condescending pity and concern, taking his arm and leading him back into the street with the Izzy demon following after.

"I'm sorry, Izzy, he'll be okay. He just needs some rest." The demon looked properly upset and nodded, but Alec could see the amusement in her cold eyes.

Jace lead them back to the institute, pulling Alec along as they stepped through the door. As soon as Izzy tried to enter, however, alarms started blaring and Raj came rushing to the door with a blade in hand.

"Where's the demon? The alarm system said there was one trying to enter."

Jace looked confused but Alec just looked towards Izzy and started yelling.

"It's her! She switched with Izzy in the alley somehow, we have to find the real Izzy."

Raj looked to the demon and then to the cuffs on Alec's wrists with a confused frown. Jace shook his head and shoved Alec inside the door.

"Alec hit his head, I'm taking him to the infirmary. Is it the sensors again? You know they can be sensitive it's probably just the ichor on Izzy's shoes."

Alec looked down at her heels, now splashed in demon Ichor.

"They weren't like that before! It's wearing a glamour!"

Jace patted his chest.

"Alec, be quiet. It's okay, buddy, we'll get you sorted out soon."

Raj took the side of the calm shadowhunter instead of the raving one in handcuffs, nodding as he started tapping things on a tablet.

"I'll let her in manually. Go ahead guys, sorry Izzy."

Izzy nodded stiffly and hesitantly entered the institute, smiling widely when she passed through the doors unharmed.

Alec launched himself towards her, ready to rip into her, before Jace caught him and shoved him towards the infirmary. Alec may have been bigger than Jace but without his arms it was impossible to throw him off. He was man handled all the way to the infirmary and into a bed as he continued to yell about the demon.

"That is not my sister! You think I wouldn't know the difference between Izzy and a demon?"

Jace looked pained and upset as he pushed Alec into the bed, linking his handcuffs to the metal frame. Alec had to admit he probably looked insane, handcuffed and screaming about an imposter that everyone but him was being fooled by. He could feel the blood trickling down his neck from the head wound and admitted it probably wasn't helping his case.

"It's okay, Alec, everything's all right. I'm gonna take care of you, just sit still."

Suddenly, thankfully, the institute doors burst open and the real Izzy came running through. She was covered in dirt and scratches but held her blade in her hand as she came tearing towards the demon imposter standing behind Jace.

Everyone's eyes in the institute turned towards the two as they collided, crashing to the floor with a thump. Now that there were two of them it would seem fairly obvious that the one that came through the doors unimpeded would be the real Izzy and the one that set off alarms would be the fake, but the problem was that demons could change glamour's very quickly.

As soon as real Izzy tackled the fake one, the demon shifted and took the exact, messy, slightly injured, appearance of real Izzy. Now, even Alec couldn't tell them apart.

Jace stood and stared at the two in horror.

"What the hell?"

Alec simply kicked his feet and watched his sister and her double tumble on the floor.

"I fucking told you!"

No one knew what to do now that they couldn't tell the two apart, until Jace threw himself between them and shoved them away, holding them at arm's length and looking between them with horror.

"I…I don't know who's who."

Both let out choruses of "I'm the real one!" which really didn't help the situation at all.

Jace looked from one to the other but as much as he tried they looked exactly the same.

Alec looked between them with careful eyes, noting the way each held themselves, the placement of their feet, the way they scowled at each other.

He knew which was the real Izzy. He'd know her in a room of a hundred other Izzy's.

He pointed with his foot, stretching it to the Izzy on the left.

"That one's the demon, kill it!"

The left Izzy gasped and looked offended and scared while the other sighed in relief. But Jace still couldn't tell.

"How can you tell?"

Alec wished his hands were free so he could simply do it himself.

"Can't you just trust me?"

Jace looked apologetic but he couldn't exactly trust someone who may or may not have a concussion.

"I don't want to accidently kill our little sister! I'm gonna need some proof, man."

Alec groaned and rolled his eyes, extremely impatient with being treated like an insane person.

"That one isn't planting her feet wide enough, Izzy used to do that when she we were little and Mom would tell her off for it every day."

The real Izzy smiled at Alec while Jace only looked more confused.

"Until you taught me that stupid rhyme to help me remember. I hated it but it worked."

Alec smiled at his little sister, laughing at the ridiculous line he'd taught her.

"Plant your feet wide to fight with pride."

Jace still looked confused but swung his seraph blade towards the imposter. It screamed in rage as it came closer, morphing into its true, disgusting form, right before it's head was cut off.

The real Izzy ran towards her older brother and uncuffed him, so that he could wrap her up in a hug.

"Are you okay? Did it hurt you?" He released her and scanned her for injuries and she shook her head.

"Just a few scratches. It dragged me down the alley when we were fighting those Shax demons, and took me to an old pizzeria where it's hideout was. I only got out because it's terrible at tying knots."

She laughed and hugged her brother again.

"How did you know it wasn't me? Before I got here, I mean."

Alec looked towards the pile of demon ash on the floor.

"It tried to act like you but it wasn't the same. I could just tell. Big brother's instincts."

Jace came over and hugged his sister before smiling apologetically at Alec, who was looking at him with raised eyebrows and crossed arm's.

"Um, sorry Alec. I should have believed you."

"Yeah, you really should have."

Jace rolled his eyes and laughed.

"Oh, come on. You were acting crazy! Of course, I'd think you were insane for pulling a sword on our sister if your screaming about it like a lunatic. You should have been more civil."

"Oh really? What like, dear brother, I do believe our sister has been abducted and replaced with an impostor. Would you like to kill it with me?"

Jace laughed at Alec's awful imitation of a posh English accent.

"I could do without the smart ass right now."

"Oh, like you had a hard day? I was abducted by a demon, Jace. I think I win."


End file.
